1. Field of the Invention
The present invention relates to an electric signal outputting apparatus, a semiconductor laser modulation driving apparatus, and an image forming apparatus.
2. Description of the Related Art
In recent years and continuing, the band of signals output (output signal band) from an electric signal outputting apparatus (e.g. output driver of an integrated circuit) has drastically increased to a high speed along with, for example, the forming of fine-sized integrated circuits and the use of serial signal transmission methods. As the speed of the output signal bands increases, the electric signal outputting apparatus (e.g. output driver) is expected to fulfill more requirements.
For example, since it becomes difficult to accurately transfer data when the rise or drop of a signal wave is slow or when the jitter of a reference clock is large, it is desired for the electric signal outputting apparatus (e.g. output driver) to increase the speed of the rise or the drop of the output signal as much as possible and to generate an output signal having a waveform as close as possible to the shape of a square wave. However, in a case where the output impedance of the output side does not match the impedance of the transmission line (more specifically, in a case where the input impedance of the receiver side does not match the characteristic impedance of the transmission line), the waveform of the output signals may become distorted by reflection caused when increasing the speed of the frequency components included in the output signals. Such distortion in the waveform of the signals may adversely affect data transmission. Therefore, the output impedance of the output side and the impedance of the transmission line are to match each other in order to increase the speed of the signal output.
However, even in a case where a match in the impedances is obtained, the increase in the speed of the output signal band may cause energy loss due to, for example, dielectric loss. Taking the energy loss into consideration, there is a method referred to as “emphasis” where the output voltage amplitude of data is emphasized beforehand when the output is switched. For example, Japanese Laid-Open Patent Application No. 2002-325019 discloses a configuration of an emphasis circuit.
However, as the speed of signal transmission further increases, a large amount of time is consumed in charging and discharging the capacitance of the transistor or diodes for electrostatic protection or the parasitic capacitance of output nodes. As a result, it is becoming more difficult to increase the speed of the rise and drop of output signals. Furthermore, the above-described emphasis method disclosed in Japanese Laid-Open Patent Application No. 2002-325019 is used for the purpose of compensating for high frequency loss, and not for increasing the speed of the rise and/or drop of signals. Therefore, the emphasis method disclosed in Japanese Laid-Open Patent Application No. 2002-325019 is limited to increasing the speed of the rise and/or drop of signals only to a certain extent.